In the related art, a variable reluctance sensor (VRS) is described in U.S. Pat. No. 5,726,888, which discloses a method and system for testing a wheel speed sensor input circuit in an anti-braking system (ABS) and/or traction control (TC) system. Alternatively, a VRS, such as the VRS manufactured by Electro Corp., may be used to measure engine speed.
The conventional VRS is a coil of wire wrapped around a permanent magnet, with leads for connection into a circuit. Typically, the VRS is in the form of a head that is positioned adjacent a rotatable gear or the like. Thus, when the ferrous metal of a tooth of a gear moves past the sensor head, the magnetic field is altered, and a voltage is induced in the coil. The amplitude of the induced voltage can vary based on factors such as, for example, the gap between the VRS and the gear tooth and the speed at which the gear tooth passes the VRS.
As the metal of the gear tooth approaches the VRS, the voltage increases. When the center of the gear tooth passes across the VRS, however, the voltage equals zero; the voltage then reverses as the gear tooth moves away from the sensor. Accordingly, the voltage output of the VRS appears as a sinusoidal wave, and points at which the wave crosses zero volts from a peak voltage represent the points at which the center of the gear tooth passes the VRS. A comparator is used to determine whether a logic condition of 0 exists (conventionally, when the VRS output voltage is 0V) or 1 exists (conventionally, when there is a VRS output voltage greater than zero, e.g., 0.5V during a start-up phase of engine operation). Typically, the portion of the sinusoidal wave that is less than zero is not used by the VRS.
However, the related art VRS has various problems and disadvantages. For example, but not by way of limitation, during low-voltage operation periods such as engine startup, VRS output voltage is relatively low (e.g., about 0.5V, as noted above) due to the slow speed at which the gear tooth passes the VRS. The zero-crossing voltage in such a case is approximately 0.1V. Typically, there is substantial high-frequency electrical noise produced by other engine components (e.g., firing injectors or local power supplies on-board the engine). Thus, at low speeds there is a low signal-to-noise ratio, and the voltages induced by the noise sources will interfere with the operation of the VRS. As a result, engine speed cannot be monitored in an accurate manner during the startup period and during other low operating speed periods. However, once typical engine speed is reached, the VRS output voltage increases to much higher levels, e.g., about 22 volts; thus, the signal-to-noise ratio increases sufficiently such that engine characteristics (e.g., engine speed) can be accurately measured.
The present invention is directed to overcoming one or more of the problems as set forth above.